1. Field
The disclosed concept pertains generally to electrical switching apparatus and, more particularly, to electrical switching apparatus, such as arc fault circuit interrupters. The disclosed concept also pertains to power distribution systems including arc fault circuit interrupters.
2. Background Information
Electrical switching apparatus include, for example, circuit switching devices; and circuit interrupters, such as circuit breakers and network protectors. Electrical switching apparatus such as circuit interrupters and, in particular, circuit breakers of the molded case variety, are well known in the art. See, for example, U.S. Pat. No. 5,341,191.
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition, a short circuit or another fault condition, such as an arc fault or a ground fault. Molded case circuit breakers typically include a pair of separable contacts per phase. The separable contacts may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to a detected fault condition. Typically, such circuit breakers include an operating mechanism, which is designed to rapidly open and close the separable contacts, and a trip unit, which senses a number of fault conditions to trip the breaker automatically. Upon sensing a fault condition, the trip unit trips the operating mechanism to a trip state, which moves the separable contacts to their open position.
Industrial circuit breakers often use a circuit breaker frame, which houses a trip unit. See, for example, U.S. Pat. Nos. 5,910,760; and 6,144,271. The trip unit may be modular and may be replaceable, in order to alter the electrical properties of the circuit breaker.
It is well known to employ trip units which utilize a microprocessor to detect various types of overcurrent or other fault conditions and provide various protection functions, such as, for example, a long delay trip, a short delay trip, an instantaneous trip, an arc fault trip or a ground fault trip. The long delay trip function protects the load served by the protected electrical system from overloads and/or overcurrents. The short delay trip function can be used to coordinate tripping of downstream circuit breakers in a hierarchy of circuit breakers. The instantaneous trip function protects the electrical conductors to which the circuit breaker is connected from damaging overcurrent conditions, such as short circuits. The arc fault trip function protects the electrical circuit from series and/or parallel arc faults. As implied, the ground fault trip function protects the electrical circuit from faults to ground.
The over current trip coordination between circuit breakers can be achieved by zone interlocking in which a circuit breaker lower in the protection hierarchy sends a fault detection signal to the upstream circuit breaker to temporarily desensitize the trip curve in a region (e.g., short delay) of the upstream circuit breaker, giving the downstream circuit breaker time to respond.
During sporadic arc fault conditions, the root-mean-squared (RMS) value of the fault current is too low to activate the thermal-magnetic trip mechanism. A conventional circuit breaker will therefore not trip. The addition of electronic arc fault sensing to a circuit breaker can provide the protection for a sputtering arc fault. Ideally, the output of an electronic arc fault sensing circuit directly trips (i.e., opens) the circuit breaker. See, for example, U.S. Pat. Nos. 6,710,688; 6,542,056; 6,522,509; 6,522,228; 5,691,869; and 5,224,006.
Arc faults can be series or parallel. Examples of series arcs are: a broken wire where the ends of the broken wire are close enough to cause arcing, or a loose or relatively poor electrical connection. Parallel arcs occur between conductors of different potentials or between the power conductor and a ground. Unlike a parallel arc fault, series arc faults almost never create an increase in current since the fault is in series with the load. In fact, a series arc fault results in a reduction in load current and cannot be detected by the normal overload and overcurrent protection of conventional protection devices. Even the parallel arc, which can draw current in excess of normal rated current in a circuit, produces currents which can be sporadic enough to yield RMS values less than that required to produce a thermal trip, or at least delay operation. Effects of the arc voltage and line impedance often prevent the parallel arc from reaching current levels sufficient to actuate the instantaneous trip function. In addition, damage from such an effect is aggravated by the concentrated nature of such faults.
A coordinated trip response to fault conditions is desired to assure that the circuit breaker closest to the fault trips opens while the upstream circuit breaker does not trip, thereby minimizing the disturbance to the whole system by isolating the fault.
For arc fault circuit interrupter coordination, it is desired to isolate the arc fault (minimize the system disturbance). However, there is no known coordinated arc fault circuit interrupter that provides arc fault trip coordination between upstream and downstream arc fault circuit interrupters.
There is room for improvement in electrical switching apparatus, such as arc fault circuit interrupters.
There is also room for improvement in power distribution systems including arc fault circuit interrupters.